The majority of suspension systems for existing surgical lighting systems utilize mechanical radial braking devices in order to provide the required rotational performances of system components. The basic principle of these braking devices is that the force needed to achieve a desired level of friction/braking is applied in the radial direction (i.e., perpendicular to the axis of component rotation).
FIG. 1 illustrates an exemplary existing suspension system 10 for a surgical lighting system. Suspension system 10 is generally comprised of spindles 12 and arms 20. Spindles 12 include a mounting tube or ring 14. A first arm 20 includes a mounting hub 22 for mounting of the arm to a spindle 12. Arms 20 may also include a joint 24. A lighthead 35 is shown mounted to fourth arm 20.
FIGS. 2 and 3 illustrates an existing prior art brake screw 40 (i.e., M10×1 brake screw) used by several manufacturers of surgical lighting systems. Brake screw 40 is an assembly generally comprised of a screw member 42 (e.g., a steel screw member), a brake member 62 having a braking surface 64, and a bias member 72.
Screw member 42 includes a head section 44 and a rivet section 54. Head section 44 has a threaded outer surface 46 and a slot 48 dimensioned to receive a flathead screwdriver for rotational adjustment of brake screw 40. Rivet section 54 is integrally formed with screw member 42. Brake member 62 is ring-shaped, and thus there is an opening in the center of brake member 62. In the illustrated example, brake member 62 is made of a copper alloy material. Bias member 72 may take the form a biasing element, such as Belleville washers or a metal spring.
Brake screw 40 is available in several different sizes and is used to control rotational braking of various components of a surgical lighting system (e.g., horizontal/extension arms, spring arms, lightheads, monitor yokes, etc.).
As best seen in FIG. 2, brake screw 40 is installed into a mounting hub 22 of an arm, and is tightened to generate a radial/normal force against a curved surface 16 of a stationary mounting tube or ring 14 (or other stationary component of the suspension system). Initial contact between braking surface 64 of brake member 62 and curved outer surface 16 of mounting tube or ring 14 is a line contact. This requires a significant amount of radial force to achieve the frictional force large enough to prevent drifting of the rotating component (i.e., to balance the external moment around an axis of rotation). Overtightening and wearing of brake member 62 eventually causes rivet section 54 of brake screw 40 to become exposed. As a result, a surface of rivet section 54 comes into direct contact with curved outer surface 16 of mounting tube or ring 14. Since rivet section 54 has a smaller contact surface than braking surface 64, overtightening can cause rivet section 54 to dig into mounting tube or ring 14. Since the material of screw member 42 is harder than the material of mounting tube or ring 14, screw member 42 causes damage to mounting tube or ring 14, which is typically not a field replaceable/serviceable component.
It should be noted that braking surface 64 of brake member 62 is located very close to the tip of rivet section 54. When braking surface 64 of brake member 62 wears to the tip of rivet section 54, brake screw 40 becomes non-functional.
In the existing prior art brake screw 40, the area of braking surface 64 is limited due to the riveted method of attaching brake member 62 to screw member 42. This attachment method requires an opening at the center of braking member 62, which increases brake pressure, thereby accelerating wear of braking member 62.
Even when not overtightened, the linear wear limitation of brake member 62 is controlled by the thickness of the braking member 62 from braking surface 64 to the end of rivet section 54. In the case of brake screw 40 illustrated herein, this thickness is only 0.8 mm (0.031 in).
In view of the foregoing, there is a need for an improved brake screw that overcomes the drawbacks of prior art brake screws and has an extended useful life.